Merged two DecimalValidator tests together

These two tests were previously added in
https://github.com/tomchristie/django-rest-framework/commit/7d79cf35b7be01b175d8c25276a4414e8144a16b
but we have now discovered that there are not actually two separate
cases, there was just a  bug in the code that made it look that way.

This also removes a redundant check to see if `DecimalValidator` was
defined.

rest_framework/utils/field_mapping.py

@@ -133,7 +133,7 @@ def get_field_kwargs(field_name, model_field):
     if isinstance(model_field, models.DecimalField) and DecimalValidator:
         validator_kwarg = [
             validator for validator in validator_kwarg
-            if DecimalValidator and not isinstance(validator, DecimalValidator)
+            if not isinstance(validator, DecimalValidator)
         ]
 
     # Ensure that max_length is passed explicitly as a keyword arg,

tests/test_model_serializer.py

@@ -22,7 +22,7 @@ from django.utils import six
 
 from rest_framework import serializers
 from rest_framework.compat import DurationField as ModelDurationField
-from rest_framework.compat import DecimalValidator, unicode_repr
+from rest_framework.compat import unicode_repr
 
 
 def dedent(blocktext):
@@ -872,25 +872,9 @@ class DecimalFieldModel(models.Model):
 
 
 class TestDecimalFieldMappings(TestCase):
-    @pytest.mark.skipif(DecimalValidator is not None,
-                        reason='DecimalValidator is available in Django 1.9+')
-    def test_decimal_field_has_no_decimal_validator(self):
-        """
-        Test that a DecimalField has no validators before Django 1.9.
-        """
-        class TestSerializer(serializers.ModelSerializer):
-            class Meta:
-                model = DecimalFieldModel
-
-        serializer = TestSerializer()
-
-        assert len(serializer.fields['decimal_field'].validators) == 2
-
-    @pytest.mark.skipif(DecimalValidator is None,
-                        reason='DecimalValidator is available in Django 1.9+')
     def test_decimal_field_has_decimal_validator(self):
         """
-        Test that a DecimalField has DecimalValidator in Django 1.9+.
+        Test that a `DecimalField` has no `DecimalValidator`.
         """
         class TestSerializer(serializers.ModelSerializer):
             class Meta: